With the evolution of transmission and amplification technology, it has become possible for a modern transport network to carry high-speed optical signals for distances of up to 3000 km or more without requiring regeneration. This reach exceeds the spacing between points where it would be convenient to switch or cross-connect the individual wavelength channels, in both metropolitan and long-haul environments. In a metropolitan environment, a convenient distance between switching nodes might typically be 20–50 km, whereas in a long-haul environment it might typically be 60–500 km.
In an optical transport network, it is of importance to verify the integrity of connections made by each switching node, either as a means to build up an end-to-end connectivity check or to provide fault location and diagnosis in the event that an end-to-end check indicates a problem. Accordingly, it is desirable to embed connectivity information within each wavelength channel and then to extract and process this information at the intermediate switching nodes. It is also beneficial to provide each wavelength channel with the capability to carry additional information such as the wavelength origin, the wavelength destination, intermediate routing information, requests for new switched paths, the payload type, the level of service, the level of protection, verification information to permit link quality assessment, a wavelength serial number, etc.
All of the above information can effectively be handled by an overhead bit stream having a bit rate of anywhere from a few kbps to several Mbps. One possible technique for transmitting this overhead information is to endow a payload bit stream with a “wrapper” signal so that the resulting composite signal, which occupies a particular wavelength channel, transports both payload and overhead information related to that wavelength channel. Conventional wrapper signals typically fall into two categories, namely digital and analog.
For instance, one way of transforming a payload bit stream and an overhead bit stream into a composite signal using a conventional digital wrapper is now described with reference to FIG. 1A and FIG. 9. A payload bit stream 100 arrives at a payload buffer 910. The payload bit stream 100 may be any type of digital data stream at the appropriate bit rate. In a SONET environment, the payload bit stream 100 may include either or both of the “SONET payload” and the “SONET overhead” of the SONET signal. In the following, therefore, the expression “payload bit stream” refers to a line signal in a broader sense and its meaning should not be confounded with the meaning of the expression “SONET payload” used to describe part of a SONET signal.
A clock source 920 provides a write clock to the payload buffer 910 at a rate equal to the bit rate of the payload bit stream 100. The output of the payload buffer 910 is read using a read clock at a gapped clock rate, resulting in the creation of a “gapped” bit stream 120 with recurrent gaps 130A–C. The gapped bit stream 120 is fed to an interleaver 930.
Meanwhile, an overhead bit stream 140 (containing the overhead information) arrives at an overhead buffer 940. The clock source 920 provides the overhead buffer 940 with a steady write clock at the overhead bit rate. The output of the overhead buffer 940 is read using a read clock at a “bursty” clock rate, resulting in the creation of a wrapper bit stream 150 composed of recurring wrapper segments 150A–150C. This wrapper bit stream is also fed to the interleaver 930.
At the interleaver 930, each wrapper segment in the wrapper bit stream is inserted into a gap in the gapped bit stream 120. An accelerated clock provided from the clock source 920 can be used to select the appropriate port from which the interleaver 930 is currently reading. By coordinating the acceleration of the overhead bit stream 140 with the acceleration of the payload bit stream 100, the wrapper segments 150A–C can be made to fit precisely into the gaps 130A–C of the gapped bit stream 120. The result is the creation of a composite signal 160 with alternating payload and wrapper segments and having a bit rate which is the sum of the bit rates of the original payload bit stream 100 and the overhead bit stream 140.
A forward error correction (FEC) block 950 is sometimes connected to the output of the interleaver 930 in order to add redundancy to the composite signal 160 prior to transmitting it over an interface.
It should be apparent that in order to recover the payload bit stream 100 and the overhead bit stream 140 at an end node, the receiver used at the end node must be operable at a speed which is sufficiently high to handle the total bit rate of the composite signal 160. Since the composite signal 160 has a bandwidth which is only slightly higher than that of the payload bit stream 100, there is practically no cost increase associated with meeting the detection requirements at the end node when compared with the case where no digital wrapper is used.
However, the overhead information in the overhead bit stream 140 needs to be extracted not just at an end node but also at multiple intermediate nodes along the way. Because the wrapper bits are embedded within the composite signal 160 travelling at the accelerated bit rate, the receiver used at an intermediate node must have the same bandwidth as the receiver used at the end node. In addition, the receiver used at the intermediate node must operate at a sufficiently low bit error rate (BER) to enable the extraction of a reliable overhead bit stream. In practice, modern high-speed transmission systems rely on high-gain forward error correction (FEC) coding systems to achieve this low bit error rate and hence an FEC decode of the entire coded signal (payload and wrapper) would be required at the receiver.
Unfortunately, not only are high-speed, low-BER receivers complex and expensive, but their use at an intermediate node raises issues of network security. Specifically, a switching node that is designed to extract the individual wrapper bits from a composite signal employing a conventional digital wrapper has the inherent capability to “eavesdrop” on the associated payload bit stream. Hence, the use of high-speed receivers at an intermediate node is highly undesirable.
In addition, since the receivers are bit-rate/protocol sensitive, it is necessary to deploy multiple such receivers in parallel at any node which purports to handle a large number of bit-rates and protocols or which is claimed to be protocol independent. Since such receivers require a high level of input optical signal, the input to them would need to be switched and/or amplified.
FIG. 1B illustrates an alternative method of carrying overhead information, which reduces amplification requirements and permits the use of low-speed receivers at an intermediate node. Therein is shown a composite signal 165 resulting from low-intensity modulation of the payload bit stream 100 with a narrowband “analog” wrapper signal 155. The wrapper signal 155 can be a line coded analog signal derived from the signal level of the overhead bit stream 145. Those skilled in the art will appreciate that it is possible to extract the wrapper signal 155 at an intermediate node by accessing the composite signal 165, converting it to electrical form using a low-bandwidth receiver, followed by detection and decoding.
However, the analog wrapper approach has severe limitations of its own. For instance, the superimposition of an intensity-modulated wrapper signal 155 on the payload bit stream 100 results in partial closure of the data “eye” at the end receiver, due to the variation in the signal level of wrapper symbols seen by the receiver. This in turn has the effect of reducing the maximum distance before regeneration of the optical signal is required unless “slow” modulation is applied, in which case the D.C. wander of the composite signal 165 can be compensated for by an automatic gain control circuit at the end receiver. However, slow modulation is associated with a reduction in the information capacity of the overhead bit stream.
In practice, amplifier and receiver automatic gain control loops typically have settling times of 50 microseconds to 1 millisecond, restricting the analog modulation rate to 1–20 kHz if there is to be little or no data eye. This is very limiting and hence systems are usually designed to allow some data eve closure by re-rating the system reach.
Thus, using conventional analog or digital wrapper techniques, it is not possible to provide a sufficiently high-capacity overhead channel on each wavelength channel in such a way that the system reach is not significantly eroded (e.g., beyond a few percent of the maximum in the absence of a wrapper) while permitting the use of low-bandwidth receivers at an intermediate switching node for extraction of the overhead information.
The shortcomings of conventional wrapper-based techniques become even more apparent in the context of a so-called “photonic” switching network, wherein all or part of the switching is performed purely in the optical domain, so that there is very little, if any, conversion of the optical signal back to electronic form. By relaxing the opto-electronic conversion requirements, the use of photonic switching not only reduces costs but also permits de-coupling of the distance between switching points from the distance between electro-optic conversion points.
A generic example of a photonic switching node is now described with reference to FIG. 7, wherein is shown a switching module 720 for switching the optical wavelength channels arriving on a plurality of input optical fibers 710. Each of the input optical fibers 710 carries a wavelength-division multiplexed (WDM) input signal consisting of a plurality of individual composite signals on distinct wavelengths. Each composite signal can be said to occupy a respective wavelength channel and may be associated with any one of a number of different payload bit rates. The switched wavelength channels at the output of the switching module 720 are recombined into WDM signals and provided on a plurality of output optical fibers 740.
The switching module 720 contains a photonic switching core 722 and, optionally, a wavelength converting switch 724 connected thereto. The photonic switching core 722 receives WDM signals on the input optical fibers 710 and on a plurality of optical fibers 730 arriving from the wavelength converting switch 724. The signals carried by the optical fibers 730 may or may not be WDM signals. The photonic switching core 722 comprises optical components for separating any WDM input signals present on the optical fibers 710, 730 into their individual wavelength channels.
The photonic switching core 722 also comprises optical components for switching the separated wavelength channels entirely in the optical domain by using any known optical switching technology, for example arrays of micro-mirrors moved or steered by a micro-electro-mechanical structure, for example.
At its output end, the photonic switching core 722 comprises optical components for recombining groups of switched wavelength channels into WDM output signals which are then output onto the plurality of output optical fibers 740. Other switched wavelength channels are fed back into the wavelength converting switch 724 along a plurality of optical fibers 750. The photonic switching core 722 may comprise optical components for combining individual switched wavelength channels into WDM signals before providing them to the wavelength converting switch 724 along the optical fibers 750.
Those skilled in the art will appreciate that the normal “through path” for a wavelength channel as it travels across a photonic network is from the appropriate input fiber 710 through the photonic switching core 722 to the appropriate output fiber 740 without interaction with the wavelength converting switch 724. However, if onward propagation is blocked at the original wavelength or because optical impairments have built up to the point where regeneration is necessary, then the wavelength channel may pass through the wavelength converting switch 724.
Also, the wavelength converting switch 724 is used for providing an optical line signal at its launch point in the first place or for terminating it at its destination. Thus, a plurality of launch signals, possibly in WDM format, arrive at the wavelength converting switch 724 along a plurality of “add” optical fibers 760. In addition, a plurality of terminated signals, possibly in WDM format, leave the wavelength converting switch 724 along a plurality of “drop” optical fibers 770. The add and drop signals could be optical WDM, optical short reach (such as parallel optical interface, e.g., Infineon PAROLI) or electrical.
Structurally, the wavelength converting switch 724 provides optical components for separating any WDM signals present on the optical fibers 750, 760 into their individual wavelength channels, as well as electrical circuitry for electrically switching these channels. Some of the switched wavelength channels are provided to the photonic switching core 722 along the plurality of optical fibers 730. Other switched wavelength channels are provided as “drop” signals to an external destination via a plurality of optical fibers 770.
To extract overhead information at the switching node, the input optical fibers 710 are intercepted by a respective plurality of optical tap couplers 780. The signals tapped in this way are WDM optical signals. The individual wavelength channels in each tapped WDM optical signal can then be isolated using a front end 790. At the output of the front end 790, there is connected an information extraction system bank 792 which includes multiple format-specific opto-electronic receivers whose electrical bandwidth of operation depends on the bandwidth of the wrapper signal carrying the overhead information. The information extraction system bank 792 is connected to a processing module 796.
Various possible implementations of the front end 790 are described in the above-referenced co-pending applications, namely Ser. No. 09/580,495 and Ser. No. 60/207,292. However, not all of these front end configurations would be suitable for use with signals which carry overhead information. Specifically, since it may be desirable to provide continual monitoring of overhead information on each wavelength channel, the most suitable configurations of the front end 790 are those which provide separation of each tapped WDM signal into its individual wavelength channels.
Regarding the information extraction system bank 792, a sufficient number of information extraction systems (such as the one shown at 1000 in FIG. 10) must be provided in order to extract the overhead information for each individual wavelength channel. Each comprises a receiver portion 1005 connected to an FEC decode block 1040, which is connected to a high-speed frame find module 1050 and a high-speed wrapper expander 1060.
When used with conventionally wrapped digital signals, the information extraction system 1000 presents a number of disadvantages which are now described. Firstly, in a practical multi-purpose multi-bitrate networking scenario, it is highly unlikely that the wrapper information on each wavelength channel will have a common bit rate, since the received wrapper data rate is largely dependent upon the payload data rate (which may range from OC-3 at 155 Mbps to OC-768 at nearly 40 Gbps and above). Therefore, there must be a sufficient number of format-specific receivers 1010 in the information extraction system 1000 to accommodate the various wrapper information bit rates.
Secondly, the information contained in a conventional digital wrapper must be received at the bit rate of the composite signal, which can be very high (in the multi-Gpbs range, for example). Thus, the format-specific receivers 1010 in the information extraction system 1000 must operate at a very high bandwidth and such receivers are more expensive than their narrower-bandwidth counterparts.
Moreover, the complexity of the front end increases because now the front end 790 must output the wavelength channels to the appropriate receiver 1010 based on the bit rate of that channel's payload bit stream. Hence, a demultiplexer 1020 and a multiplexer 1030 would be required. Also, the system 1000 is inflexible as additional receivers would be required in the receiver bank 792 for each newly introduced composite signal bandwidth.
Finally, due to the configuration of the front end 790, which provides one signal for each wavelength channel, a very large number of information extraction systems 1010 (each with multiple format-specific, high-bandwidth receivers) would be required.
On the other hand, the use of an analog wrapper in a photonic switching scenario is also impractical for various reasons, most notably due to the extreme limitations in communication bandwidth noted earlier if link performance penalties are to be avoided, as well as the presence of the wavelength converting switch 724. Simply put, the overhead information carried by an analog wrapper signal forming part of a given composite signal will be lost if the signal passes through the wavelength converting switch 724, unless the analog wrapper is detected at the input of the wavelength converting switch 724 and then re-superimposed on the appropriate signal at the output of the wavelength converting switch 724. However, the added complexity of performing this operation is a disadvantage which compounds the disadvantages arising from the analog wrapper's generally low information capacity.
It should therefore be apparent that the problems associated with carrying and extracting overhead information using conventional analog and digital wrapper techniques in a high-speed transport network are exacerbated when the transport network is equipped with photonic switching nodes.